This invention relates to liquid metal cooled fast breeder nuclear reactors and to fuel element sub-assemblies therefor.
In fast breeder nuclear reactors it is common to divide the fuel assembly into replaceable fuel element sub-assemblies each comprising a bundle of fuel pins contained within a wrapper or shroud through which liquid metal is flowed in heat exchange with the fuel pins. In one known construction of nuclear reactor the fuel element sub-assemblies are upstanding from a carrier associated with a core supporting diagrid and are arranged in groups and urged into leaning abutment with a central support member for each group of sub-assemblies. The upper regions of the sub-assemblies contain massive steel shielding to form, in combination, an upper shield for the core. Thus with the weight mass being disposed at the upper free end of each sub-assembly there is a tendency for vibration to be set up by coolant flow through the sub-assembly. It is expected that in one envisaged construction of reactor core the amplitude of vibration of the tip of a sub-assembly could be 1.0 mm and, apart from the deleterious effect on the mechanical reliability of the reactor core, such amplitude of vibration could create excessive reactivity noise, that is, it could create slight, variable reactivity changes.